I prefer Aliens to C3's but thats just me. The size depends on what else you've got. Whats your smallest regular cam? Then get the next size down from that. Its pointless having a full range of biggish cams, then a gap, then a very small one. In general you'll use the bigger small cams more, but will be glad to have the very small ones if you ever need them, cos usually nothing else will go. Very small cams are hard to place properly because you usually cant see the lobes properly so you dont know if its well-seated or not.

I would agree with Hector in saying I prefer the Aliens, which are 4 lobed devices as apposed to the C3's that are 3 lobed. My Aliens have been indisposable on any route I've climbed.
If you must buy C3's, which are too great camming devices, then look at getting the 0 and 1 sizes (green and red respectively). They match the sizes of Aliens that I use most often.
You might also consider getting the Metolius Ultralight TCU. They are more than $10 cheaper and have a slightly wider camming range than the C3's. They are also a little less fiddley to use and repairing damaged trigger cables is easier. Sizes 0 and 1 (purple and blue respectively) will do for a start.

Barring the dudes at customs crapping their pants, the whole lot costs US$600 (@ 6.80) = R4080. The exact same range of gear but from Metolius is $40 more expensive. Buy this whole lot from the cheapest SA supplier, and you'll spend R6997 (excluding postage, pending my ability to add correctly) and ignoring the fact that you'll have to fly a bit of weight back...